1. Field of the Invention
The present invention pertains generally to xerographic printing and copying machines, and more particularly to a system or apparatus for detecting the presence of a fire in a fusing device within the copying machine and for automatically extinguishing such fire.
In accordance with well-known techniques in the xerographic printing and copying arts, an image bearing member, on the form of a photoconductive matrix, is charged to a substantially uniform level and, thereafter, is selectively discharged to provide an electrostatic latent image pattern in accordance with graphic material on an original document to be reproduced. The photoconductive matrix becomes discharged in the areas exposed to the illumination and retains its charge in the areas not so exposed, the latter corresponding to the graphic material on the original document. The latent image is developed by depositing toner particles, thereby resulting in a visible image which is then transferred to plain copy paper on other suitable substrate. The toner particles are a heat settable color thermoplastic powder and will electrostatically adhere to the image bearing member or to the final support material, being passed through a fuser which generates heat of sufficient intensity to permanently fix the toner particles in the desired image configuration.
Since most developing or toner materials used in plain paper electrostatographic copying are formed of thermoplastic materials which melt at fairly high temperatures, it is typical in practice to utilize a fusing device having a radiant energy source of heat which generates an extremely high temperature atmosphere in the area through which a sheet of copy paper passes. The extremely high temperature is necessary because the fusing of the developing material must take place while the sheet of paper is moving through the fuser and a given segment of developing material is exposed to the source of heat for only a brief period of time. Since the temperature of the atmosphere immediately adjacent the paper exceeds the ignition temperature of the paper, it is clear that the paper will catch fire as a result of almost spontaneous combustion should, for example, the sheet of paper stop its movement within the fuser. This situation could be very dangerous since other parts of the copying machine could catch fire from the burning paper which is only partly enclosed within the fuser, thereby resulting in extensive damage. If the burning sheet were not contained within the machine, it could be transported into the catch tray which might contain copies previously deposited, whereby the tray could catch fire and possibly cause great damage to the surrounding environment as well as causing injury to the machine operator.
Accordingly, it is the primary object of the present invention to improve the safe operation of fusing devices in copying machines by efficiently detecting the outbreak of fire, such as when a copy sheet begins to burn.
2. The Prior Art
The above-described problem has long been recognized in the electrostatographic copying field as well as in the field of motion picture projection. In the latter case it is known, for example, from U.S. Pat. No. 1,845,840 to provide a restricted passageway through which the motion picture film is drawn while passing through a projector so that if the film should catch fire for any reason, the fire will be choked or snuffed out due to lack of oxygen within the confined space of the restricted passageway.
In the copying field, a variety of approaches have been taken in order to overcome the problem with fire in the fusing devices of the various types of copying machines. For example, in U.S. Pat. No. 3,357,401 an air jet detector is utilized to detect the presence of a copy sheet exiting from the fusing device. In the event that the copy sheet jams in the fusing device and fails to exit therefrom, an electric circuit responsive to the combined effects of the air jet and a timer operates to energize a fire extinguishing system.
In U.S. Pat. No. 3,705,289, a bridge circuit is utilized to detect temperature in the fuser above a predetermined limit to automatically terminate the operation of the fuser while allowing a ventilator to continue to operate. The bridge circuit continuously monitors the fuser to assure that the temperature remains within predetermined limits.
In U.S. Pat. No. 3,804,516, a similar type of electric bridge circuit is utilized to detect the presence of a fire in the fuser by measuring any variation in the temperature resulting from a burning sheet in order to generate an output signal to warn the operator of the existence of the fire in the machine.
In U.S. Pat. No. 3,748,088 a mechanical device is utilized to measure the velocity of the copy paper traveling through the fuser and any variation from a predetermined velocity is electronically sensed to activate appropriate controls to de-energize the source of heat in the fuser to prevent the copy sheet from catching fire.
In U.S. Pat. No. 3,778,222 a fusing apparatus includes a means for sensing the presence of fire, in response to which suitable mechanical means operate to enclose the passageway of the copy sheet through the fusing device thereby preventing spread of the fire from the fusing device.
Most recently in U.S. Pat. No. 3,979,161 a fusing apparatus in an electrostatographic copier includes a fire extinguishing snuffer device located somewhat downstream from the heat radiating portion of the fuser so that in the event of combustion of the copy sheet the flames will be extinguished by the copy sheet passing between closely spaced plates of the snuffing device.
Although the problem of fire in the fusing device has been met in accordance with the variety of approaches disclosed in the above-cited patents, it turns out that, although most of these approaches have merit in one way or another, they are not ideally suited to work in high ambient temperatures, nor is their response time as rapid as required.
Accordingly, it is another primary object of the present invention to provide a quick acting fire detecting and extinguishing system for a copying machine and further to provide one that is low in cost and simple to make.
Another important object of the present invention is to provide that the detection is not dependent on the rate or rise of temperature or on reaching the melting point of fusable wires or the like, but instead will operate instantaneously upon detection of flame. The principle upon which the present invention operates is the ionization of ambient air due to the presence of flame which will cause a substantial increase in conduction between a pair of flame detector electrodes.
Although the broad principle of flame ionization has been known in fire detecting systems such as, for example, in U.S. Pat. No. 2,385,976 to Evans et al, such principle has not been effectively exploited in a copying machine environment. Other examples of flame ionization detectors are those described in U.S. Pat. Nos. 3,551,908 and 3,740,574. In the last-cited patent, the system operates such that the flame itself generates an AC signal when a DC source is impressed between flame detecting electrodes and the flame monitor therein is sensitive to AC signals appearing across the flame electrodes above a certain frequency or above a certain voltage so as to indicate flame presence. However, that system is directed to continuously monitoring the desired condition, namely, the presence of a flame and for giving an alarm only in the situation where the flame does not appear. To the same general purpose in the system disclosed in U.S. Pat. No. 3,551,908, which is directed to utilizing the ionization pattern of a flame gas to modulate a low voltage carrier signal impressed between a sender electrode and a receiver electrode. The resultant modulating signal at the receiver electrode is used to gate a latching switch, such as an SCR, which actuates an output switch to a readout device so as to indicate flame presence. In this case, also, the crucial concern is the existence of a flame, the absence of such flame being abnormal and being arranged to give an alarm signal. In other words, it is flame failure that is to be indicated by an appropriate alarm.
In contrast with the above-noted patents, and in particular, U.S. Pat. No. 3,551,908 and 3,740,574, it is the purpose and objective of the present invention to continuously provide to a transmitting electrode a pulsating signal, preferably having a frequency of the order of 300 cycles per second. Such signal is received by the receiving electrode to an insubstantial degree unless and until the critical situation is present, that is, a flame surrounds the electrodes so as substantially to ionize the ambient and so reduce the impedance of the conduction path between electrodes, whereby the signal is then substantially transmitted to the receiving electrode. In this latter case, the signal received is approximately one thousand times as great as the signal flowing when the gap between the electrodes is not bridged by flame and is, therefore, not ionized. Appropriate threshold means, included as part of input amplifiers, detect whether the signal received is sufficiently great to constitute an alarm condition.
Briefly described, a primary feature of the present invention resides in a flame ionization detection system for detecting a flame in an electrostatic copying machine, such system comprising means defining a path of travel for a copy sheet; first and second electrodes operatively associated with said travel path and spaced closely adjacent thereto for detecting any ionization resulting from a flame occurring adjacent the path of travel; signal generating means coupled to said first electrode for generating pulse signals having a predetermined magnitude and frequency; signal receiving means coupled to said second electrode for receiving signals therefrom; and comparator means coupled to said signal generating means and to said signal receiving means for determining if the received pulse signals are above or below a threshold level and so correspond in all respects to the pulse signals transmitted to said first electrode that they are indeed flame indicating.
It will be appreciated from the preceding definition of the primary feature of the present invention that the combination includes in effect a verification or self-checking means; that is, a means that continuously compares the magnitude and frequency of the received signal to the transmitted signal, thereby to insure that the received signal is not a spurious signal but is indeed indicating the presence of a flame.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.